These studies address the immunobiological basis for viral clearance and viral persistence during hepatitis B and C virus infections. HLA Class I and Class II restricted T cell responses to HBV and HCV antigens are correlated with virological and clinical parameters of liver disease. These studies have led to the development of a synthetic peptide vaccine for the treatment of chronic Hepatitis B. Current studies focus on the virological and immunological basis for viral persistence during chronic Hepatitis C virus infection.